The present invention relates to mobile communications systems and, more particularly, to improving synchronization and channel estimation with extended pilot symbols.
In certain mobile communications systems, such as those designed using the EIA-TIA-136 standard, also referred to as ANSI-136, a signal is transmitted in sequential timeslots. Each timeslot comprises a sequence of symbols. In ANSI-136, the slot format includes regularly spaced pilot symbols, in addition to a sequence of synchronization symbols, inserted between data symbols. This is done so that more precise fading compensation can be attained for coherent reception. Interpolation for channel estimation has been shown to outperform decision directed channel tracking based receivers in coherent modulation as discussed in H. Arslan, R. Ramesh, A. Mostafa, xe2x80x9cInterpolation and Channel Tracking Based Receivers for Coherent Mary-PSK Modulations,xe2x80x9d Proc. IEEE Vehic. Technol. Conf., Houston, USA, May, 17-19 1999. However, channel interpolation requires use of many pilot symbols. This will increase the overhead, consequently reducing efficiency of the mobile communications system. While it is possible to reduce the number of pilot symbols, reduced pilot symbols degrade performance of timing and channel estimation.
Prior systems used sufficient number of pilot symbols for estimating flat fading channels in ANSI-136 systems. The timing is estimated by exploiting all of the available pilot symbols and the synchronization sequence. Channel estimates are obtained by least square estimation over the known pilot locations. These estimates are used to find the channel estimate values over the unknown fields via interpolation. It has been observed that three pilot symbols at each location were not enough to estimate frequency selective channels. As a result, current solutions are limited to non-dispersive channels. FIG. 1 shows the downlink slot format of a previously proposed ANSI-136 system. As is apparent, overhead is an issue, as the pilot symbols and synchronization symbols reduce the data throughput.
Alternatively, blind channel estimation, also known as self-recovering, approaches have been considered extensively to increase spectral efficiency. Blind channel estimation does not require transmission of known symbols. Among different versions of blind estimation approaches, more recently introduced joint channel and data estimation approaches have found interest in mobile communications systems. These approaches, which are optimal and require relatively few received signal samples for channel estimation, are based on the maximum likelihood criterion for jointly estimating the channel impulse response and the data sequence. However, computational complexity of the algorithms is high when the intersymbol interference (ISI) spans many symbols. The joint data and channel estimation algorithms fit least square channel estimates for every possible data sequence that could have been transmitted and corresponding to the received sequence. These applications then choose the data sequence and its channel fit that minimizes the least square""s error. In practice, such algorithms are implemented recursively and non-exhaustively using generalized Viterbi algorithm and per-survivor processing (PSP). These approaches are described, for example, in N. Seshadri, xe2x80x9cJoint Data and Channel Estimation Using the Fast Blind Trellis Search Techniques,xe2x80x9d Proc. IEEE Globecom Conf., pp. 1659-1663, December 1990; J. Lin, F. Ling, J. Proakis, xe2x80x9cJoint Data and Channel Estimation for TDMA Mobile Channels,xe2x80x9d Proc. IEEE PIMRC Conf., pp. 235-239, 1992; and K. M. Chugg, xe2x80x9cAcquisition Performance of Blind Sequence Detectors Using PSP,xe2x80x9d Proc. IEEE Vehic. Technol. Conf., vol. 2, pp. 539-543, May 1997. Basically, these algorithms maintain Kxe2x89xa71 best survivors into each state. Associated with each survivor a least square channel estimate is updated at every time instant. When K=1, it is simply the Viterbi algorithm with PSP.
The present invention provides improved synchronization and channel estimation with hypothesized pilot symbols.
Broadly, a receiver has improved synchronization and channel estimation for receiving a transmitted signal passing through a radio channel. A processor receives and samples the transmitted signal for a plurality of timing hypotheses. A channel estimator hypothesizes unknown pilot symbols at a pilot cluster of the sampled signal and estimates the radio channel of the pilot cluster for each hypothesized pilot symbol. The estimator determines an error corresponding to each hypothesized pilot symbol using the sampled signal, the estimated radio channel and the hypothesized pilot symbols and determines actual timing estimates and actual radio channel estimates by selecting the radio channel estimates associated with the hypothesized pilot symbols and the timing and hypothesis that minimizes the error.
Further features and advantages of the invention will be readily apparent from the specification and the drawings.